This invention relates to bis-(2,4-dinitrophenyl)-methyl-pyridinium compounds, the preparation thereof and with their use as pH indicators and ammonia detectors.
Remission photometry has established itself in clinical chemistry because of the simple and uncomplicated evaluation of test strips and of the savings in costs thereby achieved.
The measurements are to be carried out with devices which are as economical as possible and which, in contradistinction to spectral and linear photometers, operate in a wide spectral range. A device of this kind is described, for example, in German Pat. No. 2,056,232.
In order to ensure a sufficient sensitivity of measurement, test strips which are to be measured with these devices must contain indicators which do not display steep absorption peaks but rather have broad absorption bands.
A large portion of the absorption bands are to be adapted to the emission characteristic of the fluorescent lamps which are preferably used in such devices and thus lie above 600 nm. Furthermore, it is required of the indicators that the molecular form which is not used for the measurement only absorbs in a range which the remission photometer does not record in order to produce the largest possible color difference and thus to produce a clear measurement signal by the reaction. As a rule, this means that this molecular form must have a colorless to yellow appearance.
A test strip of this kind is, for example, described for the rapid and quantitative determination of glucose in body fluids, such as blood, serum and the like, in German Pat. No. 2,264,438.
In accordance with the above-mentioned requirements demanded of indicators for measurement in inexpensive remission photometers, these test strips contain benzidine derivatives. In their reduced form, these are colorless to yellow and thus absorb in a very low wavelength range. The oxidation products, on the other hand, absorb over a wide wavelength range extending to the region of infra-red.
For the determination of the urea content of blood, hitherto no test strips were known which combined simple handling with short reaction times and a precise remission photometric evaluation. However, especially in cases of emergency, for example in case of uraemic coma, it is essential for the therapeutic measures to be undertaken to have an analytical result which is as accurate as possible in the shortest possible time. However, even in the routine operations of a clinical laboratory, with the introduction of a remissionphotometrically evaluable urea rapid test, a marked advance would be achieved since laborious pipettings, the use of reagents which, in some cases, are of limited stability and corrosive, and long reaction times, would become unnecessary.
The development of such a urea test strip has hitherto not been possible because of the lack of suitable indicators. In contradistinction to the above-described glucose test strips, which use a redox indicator, in clinical chemistry, for the detection of urea, as color indicators in the reaction sequence, use is made of pH indicators for the detection of the ammonia formed.
Two known compounds, hexanitrophenylmethane and ethyl 2-bis-(2,4-dinitrophenyl)-acetate, admittedly possess a usable absorption but, on the other hand, have too low a solubility or too high a pK value.
Other known pH indicators cannot be utilized since either not only the acidic but also the basic form absorb over a broad wavelength range and, therefore, are remissionphotometrically not sharply separated from one another or the colored form only absorbs in a more or less steep peak so that a measurement with simple devices, which only cover a wide wavelength range, is not possible.